1. Field
Embodiments of the invention relate to electronic devices, and more particularly, in one or more embodiments, to variable gain transconductors.
2. Description of the Related Technology
Wireless systems typically include a radio receiver that processes a wireless signal to recover original information carried by the wireless signal. Radio receiver architectures typically down-convert an incoming radio frequency (RF) signal to a first intermediate frequency (IF) or a baseband signal. For example, in a superheterodyne radio receiver architecture the first IF signal further down-converted either to a second lower IF signal or to the baseband signal. In a direct conversion radio receiver the RF signal is directly converted to the baseband signal that can be processed to extract transmitted information without further down-conversion. As such, direct conversion schemes can be used to eliminate external RF/IF filters and multiple down conversion stages, which can reduce the cost of a radio receiver front end.
Radio receiver architectures of all types are often configured to include a variable gain amplifier. The variable gain amplifier allows the radio receiver to receive incoming RF signals with widely varying power levels. For example, in a wireless system, the difference between strong and weak received signals can be as much as 100 dB. On the one hand, the variable gain amplifier prevents a strong incoming RF signal from saturating radio receiver components, which leads to non-linear distortions of the signal within the radio receiver. Non-linear distortions, such as clipping, lead to processing errors that result in information loss. In a complementary manner, the variable gain amplifier amplifies weak incoming RF signals so that transmitted information can be obtained from the weak signals by the radio receiver.
The variable gain control can be implemented before or after down-conversion. Prior to down-conversion, variable gain control is typically implemented in a low noise amplifier (LNA). After down-conversion, variable gain control is typically implemented in a transimpedance amplifier (TIA). Both of these options have drawbacks. For example, for a variable gain LNA, decreasing the gain improves linearity but also increases the noise figure of the entire radio receiver, since the LNA typically dominates the noise performance in a radio receiver. By contrast, while reducing the gain of a TIA does not significantly degrade noise performance, decreasing the gain in the TIA does not significantly improve the overall linearity of the radio receiver.
Additionally, since antennas typically have single-ended outputs, it is preferable for the input of the component following the antenna in a radio receiver to be single-ended as well. However, in a direct conversion radio receiver, singled-ended signals are susceptible to even order distortion, which is a form of non-linear distortion that degrades performance. One solution is to use a balun prior to the LNA. The balun may be either on the same IC as the LNA of off chip. Another solution is to convert a signal-ended signal to a differential signal within the LNA of the radio receiver. This typically involves using inductors that increase the complexity and component count of a radio receiver. For fully integrated radio receivers, on-chip inductors occupy valuable space and induce parasitic substrate interference signals in the form of eddy currents that degrade performance of other components. Off-chip inductors add to the size, component count, heat dissipation and overall cost of a radio receiver.